waiting-stream-check

Take a waiting stream, a success continuation, and a failure continuation.If we don't find any ready suspended streams, invoke the failure continuation. If we find a ready suspended stream calculate the remainder of the waitingstream. If we've reached the fixpoint just call the thunk of the suspendedstream, otherwise call mplus on the result of the thunk and the remainderof the waiting stream. Pass this result to the success contination.